Based on preliminary studies demonstrating that the envelope gene product of the human immunodeficiency virus occurs as trimeric complexes of the subunit gp120/gp41, experiments are described in this proposal with an overall goal of identifying and characterizing the functional regions of the HIV env that signal their association to a homotrimer complex. The importance of this association to the biological activity of the glycoprotein complex is also explored. To this end, specific mutations will be introduced into conserved envelope gene coding regions and the effects of the changes in the proper assembly process will be assessed. An efficient eukaryotic expression system that utilizes the HIV LTR to drive the expression of env and chemical crosslinking will be used to determine the capability of oligomerization of the env variants. Coexpression of wild-type and env variants within the same cell, will be used to study the formation of heterotrimers. Once the formation of heterotrimers are demonstrated, they will be used in experimental studies to define the cooperativity of the subunits within a trimer during the HIV env-mediated membrane fusion process. The studies that are proposed in this grant application are aimed towards an understanding of the quaternary structure of the HIV env complex and its biological activity. The information obtained from these experiments is essential to define the mechanism of virus/cell fusion, a critical step in the penetration of the HIV virus, and may contribute to the development of improved therapeutic modalities for AIDS.